Java - 如果文本文件不存在，有没有办法打开它，如果它确实存在则附加到它？

Question

我对 Java 很陌生，我遇到了这个问题。如果 java 代码不存在，我希望它创建一个 txt 文件，但如果存在，我希望 PrintWriter 使用 FileWriter 附加到它。这是我的代码：

编辑：我试图修复我的代码，但现在我收到了 IOException 错误。我在这里做错了什么？ 编辑2：我认为我的代码是独一无二的，因为我试图让它在文件不存在时创建一个新文件，如果它已经存在则让它附加到现有文件。

import java.util.Scanner;
import java.io.File;
import java.io.FileWriter;
import java.io.BufferedWriter;
import java.io.IOException;

/**
 * Created by FakeOwl96 on 3/28/2017.
 */
public class AreaOfCircle {
    private static double PI = Math.PI;
    private double radius;
    private static double area;
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        AreaOfCircle a = new AreaOfCircle();
        System.out.print("Type in the radius of circle: ");
        a.radius = keyboard.nextDouble();
        getArea(a.radius);
        System.out.print("Name of the txt file you want to create:");
        String fileName = keyboard.nextLine();
        keyboard.nextLine();
        try {
            File myFile = new File(fileName);
            if (!myFile.exists()) {
                myFile.createNewFile();
            }
            FileWriter fw = new FileWriter(myFile, true);
            BufferedWriter bw = new BufferedWriter(fw);
            bw.write("The area of the circle is " + area + ".\n");
            bw.close();
        }
        catch (IOException e) {
            System.out.println("IOException Occured");
            e.printStackTrace();
        }
    }

    public static void getArea(double n) {
        area = n * PI;
    }
}

Answer 1

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Scanner;

public class AreaOfCircle {
    private static double PI = Math.PI;
    private double radius;
    private static double area;
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);
        AreaOfCircle a = new AreaOfCircle();
        System.out.print("Type in the radius of circle: ");
        a.radius = keyboard.nextDouble();
        getArea(a.radius);
        System.out.print("Name of the txt file you want to create:");
        String fileName = keyboard.next();
        keyboard.nextLine();
        try {
            File myFile = new File(fileName);
            if (!myFile.exists()) {
                myFile.createNewFile();
            }
            FileWriter fw = new FileWriter(myFile, true);
            BufferedWriter bw = new BufferedWriter(fw);
            bw.write("The area of the circle is " + area + ".\n");
            bw.close();
        }
        catch (IOException e) {
            System.out.println("IOException Occured");
            e.printStackTrace();
        }
    }

    public static void getArea(double n) {
        area = n * PI;
    }
}

我所做的唯一改变是 字符串文件名 = 键盘.next();来自 //keyboard.nextLine() 上面的代码对我有用。希望这会有所帮助。

Answer 2

在初始化myFile后添加以下行：

myFile.createNewFile(); // if file already exists will do nothing

Answer 3

这是文件追加行的另一个示例，如果文件不存在则创建新文件。

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
public class AppendFileDemo2 {

    public static void main(String[] args) {
        try {
            File file = new File("myfile2.txt");
            if (!file.exists()) {
                file.createNewFile();
            }
            FileWriter fw = new FileWriter(file, true);
            BufferedWriter bw = new BufferedWriter(fw);
            PrintWriter pw = new PrintWriter(bw);
            //This will add a new line to the file content
            pw.println("");
            /* Below three statements would add three 
           * mentioned Strings to the file in new lines.
             */
            pw.println("This is first line");
            pw.println("This is the second line");
            pw.println("This is third line");
            pw.close();

            System.out.println("Data successfully appended at the end of file");

        } catch (IOException ioe) {
            System.out.println("Exception occurred:");
            ioe.printStackTrace();
        }
    }
}

Answer 4

这是文件追加行的示例，如果文件不存在则创建新文件。

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
public class AppendFileDemo {

    public static void main(String[] args) {
        try {
            String content = "This is my content which would be appended "
                    + "at the end of the specified file";
            //Specify the file name and path here
            File file = new File("myfile.txt");

            /* This logic is to create the file if the
         * file is not already present
             */
            if (!file.exists()) {
                file.createNewFile();
            }

            //Here true is to append the content to file
            FileWriter fw = new FileWriter(file, true);
            //BufferedWriter writer give better performance
            BufferedWriter bw = new BufferedWriter(fw);
            bw.write(content);
            //Closing BufferedWriter Stream
            bw.close();

            System.out.println("Data successfully appended at the end of file");

        } catch (IOException ioe) {
            System.out.println("Exception occurred:");
            ioe.printStackTrace();
        }
    }
}

Answer 5

还有一个例子，这次使用try-with-resources 并使用Files class 创建BufferedWriter：

public void write(File file, String text) throws IOException {
    Path path = file.toPath();
    Charset charSet = StandardCharsets.UTF_8;
    OpenOption[] options = new OpenOption[]{
        StandardOpenOption.CREATE, // Create a new file if it does not exist
        StandardOpenOption.WRITE,  // Open for write access
        StandardOpenOption.APPEND  // Bytes will be written to the end of
                                   // the file rather than the beginning
    };
    try (BufferedWriter bw = Files.newBufferedWriter(path, charSet, options)) {
        bw.write(text);
    }
}

以上示例可在 GitHub 上进行测试。

您也可以使用Files.write 方法：

public void write(File file, List<String> lines) throws IOException {
    Path path = file.toPath();
    Charset charSet = StandardCharsets.UTF_8;
    OpenOption[] options = new OpenOption[]{
        StandardOpenOption.CREATE, // Create a new file if it does not exist
        StandardOpenOption.WRITE,  // Open for write access
        StandardOpenOption.APPEND  // Bytes will be written to the end of
                                   // the file rather than the beginning
    };
    Files.write(path, lines, charSet, options);
}